For a machine to properly perform at its designed peak, the machine must have all of its parts in proper position with respect to each other. In particular, for an impeller to provide a designed pressure rise at a designed flow rate, the blades of the impeller must be properly spaced and positioned. A device called a blade locator is used to properly position each blade on a tacking ring prior to welding to an impeller cover.
One method of validating the proper position of the blade, as well as the blade shape, is by having X, Y and Z coordinates measured on the surface of the blade by a sophisticated coordinate measuring machine. A minimum of 20 coordinate points are measured on the blade and compared against the design specification coordinate location information for that blade for a particular impeller.
This method requires a blade be affixed to the tacking ring by the blade locator and the tacking ring be removed from the blade locator device and moved into a "controlled" environment room housing the coordinate measuring machine. It takes approximately an hour to get the tacking ring located on the coordinate measuring machine, and another hour to measure the coordinates on the blade. If the measurements are within tolerance, the tacking ring is sent back to the shop environment to be mounted on the blade locator device to have the remaining blades tacked in position. If the coordinates are not acceptable, the tacking ring is still sent back to the shop environment, but the blades are removed and either rebent or repositioned or both. After this re-work is accomplished, the tacking ring is again sent to the "controlled" environment room housing the coordinate measuring machine for reinspection.
Sophisticated coordinate measuring machines are very expensive and require highly skilled, trained personnel to operate the measuring machine in a specially constructed "controlled" environment room.